1. Field of the Invention
The present invention relates to radioactive power sources and micromechanical and microelectromechanical (MEM) devices of improved efficiency.
2. Discussion of the Prior Art
Microelectromechanical (MEM) devices and microelectromechanical systems (MEMS) are being developed to accomplish a number of previously unattainable goals in systems that are smaller in volume and power consumption than systems contemplated in the prior art. Recently, it has become possible to fabricate charge and kinetic energy radioisotope energy converters in a smaller scale than previously possible. MEMS manufacturing techniques have been developed for the production of microelectromechanical systems using manufacturing technologies adapted from the manufacturing of integrated circuits and other electronic devices. Such MEMS structures may be formed of a variety of materials, including semiconductors used in integrated circuit manufacturing, such as silicon, various metals, and organic thin films. The small size of MEMS systems and the materials of which they are formed naturally offers the opportunity for the integration of these structures with integrated circuits to provide autonomous Microsystems. However, such systems still require a power source, and the utility of autonomous Microsystems has been hindered by the unavailability of suitable power sources that provide enough power while not greatly increasing the final volume of the system. Even the smallest conventional batteries may be much larger than the MEMS system being supplied with power, thus limiting the extent to which the size of the overall device can be shrunk. In addition, conventional batteries have a relatively short useful lifetime, typically on the order of days to weeks or at most months, whereas in some applications it would be desirable to have a power source capable of supplying power to the MEMS device for many months or even years. Suitable devices with sufficient lifetime would be useful for a variety of applications. For example, sensor systems placed over a large area may be utilized to monitor vibration and gas output of vehicles and report back the information to a central collection point via optical or radio frequency (RF) communications. The signals produced by the small autonomous sensors may be picked up and stored and amplified by a larger central system powered by conventional sources such as gasoline engines, fuel cells, or large batteries. Such sensors have also been proposed for use in battlefield monitoring and in commercial applications for sensing properties that affect component life such as viscosity, Young's modulus, vibration, etc. If such devices could be provided with power sources capable for operating for years or decades without replacement, the sensors could be embedded inside permanent casings such as walls of buildings, or could be utilized in space research as “microsatellites.”
One proposed approach to providing long-lived power sources is the use of radioisotopes that generate electrical power in a nuclear “battery.” Early approaches to such batteries are discussed in A. Thomas, “Nuclear Batteries: Types and Possible Uses,” Nucleonics, Vol. 13, No. 11, November 1955. One approach to electric power generation from radioisotopes is based on charge particle collection. See, e.g., G. H. Miley, “Direct Conversion of Nuclear Radiation Energy,” American Nuclear Society, 1970; L. C. Olsen, et al., “Betavoltaic Nuclear Electric Power Sources,” Winter Meeting of the American Nuclear Society, San Francisco, Calif., 1969. Most of the nuclear battery designs are based on thermal effects, in which a volume of the source is self-heated due to highly energetic particle impacts, and the heat energy is then converted to electrical energy, with a typical efficiency of 4% to 15% (a quantity determined by the efficiency of the thermo-electric converter). Although such an approach is amenable to miniaturization, the need for thermal isolation and relatively high operating temperatures makes such devices suitable primarily where high power is required and where the high operating temperature and volume of heat produced is not problematic. Further, as devices shrink in size, the surface to volume ratio increases, with large losses from radiative and convective losses. In particular, the stopping depth for electrons or alpha particles in materials is usually in the range of a few microns to several tens of microns, indicating that the charged particle collector must be at least that many microns thick. This implies that the collector cannot be scaled down to a thickness less than the decay depth since otherwise the emitted particles simply pass through. Consequently, it is unlikely that traditional thermal conversion will work in microscale devices. Another approach for converting emitted charged particles to electric power is by the creation of electron-hole pairs by ionization in a semiconductor (e.g., silicon). In a depletion region electric field, the pairs can be separated to provide electric energy. This is essentially the same principle used in solar cells, where photons cause electron-hole pair generation. An advantage of the use of particles from nuclear decay is that they create thousands of electron-hole pairs per emitted particle because of the large particle energy. However, a significant disadvantage is that the high energy of the particles damages the crystal lattice, which in turn reduces the effectiveness of the capture of more particles. Although there are ways to continuously or intermittently thermally anneal the crystal, it is unlikely that such annealing will result in a fully repaired crystal and it is a process that is difficult to utilize in devices that are in place in the field. Furthermore, because such sources depend on the use of pn-junctions, the operating temperature range of the devices is limited to about −15.degree. C. to 100.degree. C. Another approach is to generate light by the incidence of the emitted particles onto a luminescent material, and then capture the emitted light with a photocell to produce electricity. However, such an approach requires very high radioactive source levels due to the low efficiency of the incident particle-to-photon production, and the consequent absorption of the photons within the photon generating layer.
Another approach which has been considered is the use of direct charge in which charged particles, e.g., electrons, emitted from a source are collected by a collector spaced by a gap from the source, thereby building up a potential difference between the source and the collector. By increasing the gap between the source and the collector, it is theoretically possible to obtain very high voltage differences (e.g., millions of volts) due to the low capacitance between the source and the collector, but any attempt to use the power from the system to drive even a picofarad (pF) load will effectively reduce that output voltage to millivolts. Consequently, such an approach would only provide useful output voltages if the load capacitance is of the same order of magnitude as that of the source-collector capacitor. For sources having relatively low radiation flux, as generally would be the case for devices to be used in the microsensor field, the voltages that could be obtained by this approach would necessarily be quite low.
Referring again, by way of background, to U.S. Pat. No. 6,479,920, FIG. 1a illustrates the basic structure of the '920 patent. In this structure a cantilever beam carries a collector 50 to collect the electrons emitted by a terminal structure 57 containing a radioactive thin film. As the electrons are emitted, a voltage builds up between the collector and the terminal structure which electrostatically deforms the cantilever. Eventually the collector and terminal structure touch each other and the vibration of the cantilever leads to power generation by a piezoelectric assembly carried on the cantilever. The reciprocation time of the cantilever assembly is a function of the charging current, the leakage current, and the mechanical properties such as the spring constant of the composite cantilever and the initial gap.
While the '920 patent demonstrated certain operational principles, the efficiency of the structure is undesirably limited. There is a need, therefore, for a structure and methods for improving efficiency.